Among the critical characteristics of paper and other sheet materials which are important to both manufacturers and users is strength. Many different methods for measuring strength have been proposed in the past, but virtually all suffer from a great disadvantage, namely, the tests are destructive and cannot be used "on line". A number of standardized tests have been devised to provide a basis for specifications by which paper can be bought and sold, and these tests provide arbitrary but nevertheless useful strength indices for comparing the strengths of various papers. Unfortunately, all are destructive, and none can be used "on-line". The more common tests are a standardized tensile test, the so-called "STFI" compressions test, and the "burst pressure" or "Mullen" test. Pierce strength is also an important strength parameter.
In the standard tensile test, a strip of paper is held between two clamps and loaded in tension at a predetermined rate. The loading at failure is taken to be a measure of the tensile strength of the paper. There are a number of standardized procedures which have been adopted to perform this test, e.g., TAPPI Standard T404os-76 and ASTM Standard D828.
The "STFI" compression test for heavy papers is a standardized test whose procedure has been established by the Swedish Technical Forest Institute as specified by the identifiers: Scan P46 Column 83. In this test a strip of the paper to be tested is held between a pair of clamps which are moved together at a fixed rate while the compressive force is monitored. "Rupture" occurs when the compressive force passes a peak and begins to drop. The force at "rupture" is taken as the compressive strength of the paper. Other standard specifications for this test are, e.g., TAPPI 7818os-76 and ASTM D1164.
The strengths of papers as measured by the foregoing tests typically have different values depending on whether the test strip is cut in the "machine direction" or the "cross direction". The "machine direction" is generally defined by the direction of travel of the sheet when it is being manufactured by the papermaking machine. The "cross direction" is generally defined as the two directions in the plane of the sheet perpendicular to the machine direction.
A "Mullen" or burst pressure test is conducted by clamping a sample of the paper between two circular clamping rings having a specified standard inside diameter, and building up pressure on one side of the paper until the paper bursts (using a rubber diaphragm and liquid pressure). The pressure required to burst the paper is known as the "burst pressure" and is the figure often used to specify the required strength. Common burst pressure specifications are TAPPI 403os-76 and ASTM D774.
On-line determination of paper strength during the manufacture of paper can aid in producing paper which can meet the specified strength requirements. Needless to say, however, none of the above-mentioned tests lend themselves to use in connection with the continuous measurement of paper strength. Nevertheless, because of their widespread popularity, it is desirable that any method used to measure the strength of paper provides results which correlate with one of the recognized standard tests.
U.S. Pat. No. 4,291,577 teaches a system for measuring the strength of paper as it is being produced. The patent provides for measurement of the velocity of ultrasonic waves through the moving paper sheet. Based upon the velocity, the strength of the paper is determined.
The patent teaches a device having two wheels which are spaced apart from one another and which roll along the moving paper sheet or "web". The first wheel contains a transducer in the form of a rectangular button mounted on the periphery of the wheel so that as the wheel rotates the button periodically contacts the paper. With each revolution of the wheel, when the transducer contacts the paper, it receives an electrical signal from a signal generator and imparts a mechanical signal to the paper. The second wheel contains a receiving transducer substantially the same as the transmitting transducer, which also is mounted on the periphery of the wheel and occupies a small percentage of the total circumference of the wheel. The receiving transducer contacts the paper once each revolution of the wheel and receives the signal from the transmitter by picking up the ultrasound signal from the paper and converting it to an electrical signal. The system also includes a position detector to monitor the rotational position of the first wheel and to trigger the firing of the ultrasound pulse by the transmitter when the wheel is in a predetermined position. The rotation of the receiving wheel is coordinated with the transmitting wheel so that the receiving transducer is in contact with the paper at the appropriate time to receive the transmitted signal. The signal from the receiving transducer is transmitted to a metering and recording apparatus which measures the velocity of the ultrasonic waves.
Unfortunately, the system described in the patent has a number of disadvantages. Since the transmitting and the receiving transducer each contact the paper during only a small percentage of the total rotation of the wheels, the rotation of the wheels must be carefully synchronized and the transmission of the pulse must be carefully timed so that the pulse is received by the transmitter when it is in contact with the paper. Furthermore, since the transmitter and the receiver are only in contact with the paper for a small percentage of the total rotation of the wheels, a substantial portion of the paper is not subject to velocity measurement.